Multi-step bleaching sequences including four bleaching steps, and in which the final bleaching includes first and second chlorine dioxide steps with an intermediate alkaline extraction can comprise, for example, a first step with chlorine/chlorine dioxide (known as CD), a second step with alkaline extraction in which oxygen gas is added thereto (known as EO), and a final bleaching step using two chlorine dioxide steps (known as D1 and D2, respectively). However, other such bleaching sequences are known, which are finalized using two chlorine dioxide steps with or without an intermediate extraction therebetween. It is known that the alkaline extraction step carried out between the chlorine dioxide steps (D1 and D2) during the final bleaching of a chemical pulp can be simplified, for example, by elimination of the washing of the pulp subsequent to the alkaline treatment step. In that case, however, it was believed necessary to neutralize residual alkali prior to the second chlorine dioxide step (D2). This, however, results in a greater consumption of alkali or chlorine dioxide, since the pH value in the second chlorine dioxide step (D2) will not be at the optimum level.
These and other problems are solved in accordance with the present invention, which results in a surprising and substantial increase in the efficiency of the alkalization, resulting in improved bleaching compared with conventional methods as set forth above.